Ancient Crust, Aqueous Alteration, and Impact Melt Preserved in the Isidis Ba- Sin, Mars

Introduction: The Isidis impact basin was formed in ancient cratered uplands but has since experienced tectonic deformation of the impact basin rim (Nili Fossae), volcanic modification of the western basin rim (Syrtis Major Planitia), and filling of the Isidis Basin (Hesperian ridged plains and the Vastitas Borealis Formation). This wide array of post-basin geomorphic units led to the general impression that primary Isidis Basin impact deposits had been highly modified, removed, or buried by eolian, volcanic, and fluvial deposits, and general weathering processes. Global Thermal Emission Spectrometer (TES) data were interpreted to indicate the presence of both basalt and basaltic andesite in the region [1,2] but also had spectral components consistent with olivine-rich materials. [3] examined the olivine occurrences in detail and concluded that the olivine may be exposures of olivinerich igneous lithologies exposed by post-basin faulting along ring faults and graben or post-Isidis volcanic flows. [4] re-evaluated these deposits with THEMIS data and assessed a number of possible origins. They found that the majority of the olivine-rich material occurred in the form of in-situ rock, with lineaments interpreted as layering, covering an area significantly larger than previously suspected. They concluded that the evidence best supports olivine-rich basalts extruded subaerially as the source of the anomalies. Based on crosscutting relationships and the lack of olivine in fractures and graben they suggest the basalts existed at the time of the Isidis impact. Left unexamined are the specific relationships between the impact-forming event, its deposits and the olivine-rich units interpreted by [4] to be of volcanic origin. In the following sections we 1) examine the geology of the Isidis basin, and 2) assess the nature of impact basin geology, settings and formation and evolution processes, in order to assess further the relationship to the olivine-rich unit. We then bring together new Mars Express OMEGA and HRSC data to examine the correlation of the olivine-rich unit to the regional geology of the Isidis basin. Methodology: Basic reduction of OMEGA data from radiance to reflectance including atmospheric removal is performed as described by [5]. Mafic minerals are recognized and mapped on the basis of crystal field transition absorptions in the 1-2.5 μm region and procedures for this recognition and mapping have been presented previously [5,6]. The determination of mineralogy for hydrated silicates is treated by Poulet et al. [7]. A detailed investigation of phyllosilicate occurrences in this region is presented in a companion abstract [8]. We have generated mineral indicator maps for olivine, lowand high-Ca pyroxene, and phyllosilicates that exhibit a narrow absorption near 2.3 μm. These maps show the locations of regions with absorptions consistent with the presence of the minerals mapped, and the stronger the indicator the stronger the mineral feature that is mapped. We are in the process of developing detection criteria, and the maps robust where the signatures are strong but should be interpreted cautiously where the signatures are weak.